In recent years numerous advantages have been found for incorporating various rubbers, or rubber-like polymers into paving asphalts. The added rubber can give the asphalt improved tensile strength, elasticity, and ductility, and can reduce its susceptibility to cracking and disintegration due to mechanical stress and/or extreme temperature changes. The extent to which these advantages can be realized however depends upon the proportion and type of rubber which is utilized, and the extent to which it is dissolved or uniformly dispersed in the asphalt. Until very recently, the art has contemplated the use of rubberized asphalts primarily as the cement for asphalt concretes and for this particular use economics dictated that no more than about 5 weight-percent of rubber could be used. These small proportions give some benefit, primarily in the area of improved ductility (as disclosed for example in U.S. Pat. No. 3,779,964), but give little improvement in other physical properties. It was also found difficult under practical conditions to obtain a uniform solution or dispersion of rubber in the asphalt. One known procedure was the addition of a latex emulsion, but this was found to be impractical because of the necessity of flashing the water from the asphalt mixture thus formed. In addition, asphalt viscosity is often reduced adversely by aqueous latex emulsions.
In U.S. Pat. No. 3,891,585 to McDonald, some progress was made toward achieving more of the potentially available benefits from the addition of rubber to asphalts. In this patent, relatively large proportions of ground, reclaimed rubber, ranging between 25% and 33% by weight, are utilized in the asphalt mix, and the mixture is used in the form of relatively thin layers or membranes applied over old pavements, the membranes then being dressed with mineral aggregate or rock chips. The asphalt-rubber mixture was prepared by heating the granulated rubber with the asphalt at temperatures between about 350.degree. and 500.degree. F, to form a "jellied" composition which provided a coating of excellent elasticity, tensile strength and durability under adverse weather conditions. However, as acknowledged by the patentee and his co-inventor Winters in their subsequent U.S. Pat. No. 3,919,148, the use of this jellied material presented one drawback; because of its viscous nature, special equipment and techniques were required for its application. It must be applied rapidly before it sets up, and the chips or aggregate must be applied to the layer of material before it sets up, in order to insure retention of the chips. All of this imposes severe time limitations upon the mixing and application of the mixture to the pavement.
To overcome the above noted difficulty, Winters et al in their U.S. Pat. No. 3,919,148, suggest adding to the jellied rubber-asphalt mixture a small proportion of a light hydrocarbon solvent such as kerosene. This is said to bring about a temporary reduction in viscosity for up to about 1 hour, during which time the mixture can be applied to the roadway and the application of rock chips completed. After rolling the chips into place it is said that "a reaction occurs between the kerosene and the asphalt-rubber composition that results in a rapid increase in viscosity and the chips are set into place so that they are not dislodged by traffic." It is conceded that if the application of chips is delayed beyond the "set," the chips will not sufficiently adhere to the asphalt to withstand vehicle traffic.
From the foregoing, it will be apparent that even with the addition of kerosene, Winters et al are still faced with a critical and troublesome timing schedule for first forming the hot asphalt-rubber "gel," adding and homogeneously mixing the kerosene with the asphalt-rubber, applying the resulting mixture to the roadway, and then applying the chips. In most cases, all of these operations must still be carried out in less than about 1 hour, which is obviously a difficult limitation to cope with in any sizeable job. It would appear therefore that the use of kerosene merely alleviates but does not solve the basic problem acknowledged by the patentees.
In addition to the foregoing, we have found that even with the use of kerosene as described by Winters et al, other problems are encountered in using the rubber-rich mixture. Firstly, the adhesive qualities of the asphalt-rubber blend are not as good as most other liquid asphalts and it is therefore generally necessary to make a light application of a liquid asphalt first, as a tack coat to the pavement being treated, to act as a "glue" to assure that the rubberized asphalt will stick. It is also generally necessary to use clean, dry, high quality rock chips to cover the hot asphalt-rubber membrane; wet or dirty chips do not adequately adhere. These factors increase the cost of any project, and at times traffic is tied up longer due to the need for a tack coat.
Moreover, the high viscosity and gel-like nature of the rubber-asphalt mixture usually means that the hot blend will not flow readily into pavement cracks when such cracks are being sealed. This means that the cracks may be sealed over by a bridging action, or it is necessary to rout out the cracks first to widen them. Sealing is then accomplished in a separate operation, which is an added inconvenience and expense. Also, in applying membranes of the mixture by conventional spraying methods, the high viscosity of the mixture makes it more difficult to obtain an even, smooth spray pattern from the asphalt distributor bar. "Roping" or "ridging" can more easily occur, which reduces the smoothness of the pavement. It can also be unsightly and the ridges can pose tire tracking problems for a vehicle.
Finally, the use of a light hydrocarbon solvent such as kerosene in itself presents additional problems. Since kerosene generally has a flash point of about 130.degree.-150.degree. F, a fire hazard is presented. Further, some of the lighter fractions of the kerosene are released to the atmosphere, creating an air pollution problem.
We have now discovered a novel technique by which all of the foregoing problems can be avoided or at least substantially alleviated. According to our procedure, the base asphalt stock is first modified by blending therewith at elevated temperatures a minor proportion of a heavy, high-boiling, highly aromatic, high-flash-point mineral oil solvent, thereby forming a base stock to which the rubber component, in granulated and powdered form, is then added. The resulting mixture is then heated with agitation at about 300.degree.-450.degree. F for about 0.5-2 hours, to obtain a homogeneous dispersion or solution of rubber in the base stock. Under normal conditions the resulting mixture presents no fire hazard or atmospheric pollution problems, and at temperatures above about 325.degree. F retains a fluid, easily spreadable consistency for periods of at least about 12 hours or more in most cases. The resulting mixture can be spread over a roadway using standard equipment and spraying techniques to form a highly adherent membrane over the roadway, which generally requires no tack coat. Due to its relatively non-viscous consistency when applied to the roadway, cracks are filled and sealed rather than bridged over. No difficulty is encountered in obtaining an even, smooth spray pattern from the asphalt distributor bar, to lay down smooth membranes ranging in thickness between about 1/16 -inch and 1/4 inch.
In addition to the foregoing, it has been found that the added high-boiling solvent substantially increases the life, as well as the cold temperature characteristics and durability of pavement constructions made from the resulting asphalt-rubber compositions.
It was found however that when the ground rubber stock employed was composed exclusively of devulcanized and/or synthetic rubber, the cooled membranes were somewhat lacking in toughness and resiliency. But it was found further that this deficiency could be remedied by including in the rubber stock employed a substantial proportion of vulcanized, ground reclaimed natural rubber. Road testing conducted to date indicates that such suitably compounded and applied membranes retain their toughness and resiliency over extended periods of time, in a manner similar to the membranes described in the McDonald and Winters et al patents.